1. Field of the Invention
The present invention relates to systems and methods for providing cardiac flow measurement and, more particularly, to a system and method for providing measurement of the volume of blood pumped by an artificial heart.
2. The Prior Art
In the human circulatory system, blood is first pumped through the lungs, where oxygen and carbon dioxide are exchanged. It then returns to the heart and is pumped out into the systemic circulatory vessels. The blood leaves the heart through the aorta, which is typically 25 millimeters in diameter, and then flows on through progressively narrowing vessels until it reaches the capillaries, which are some eight micrometers in diameter. The blood then flows through progressively larger vessels as it moves through the venous system, until it reaches the vena cava, where the vessel diameter is about 20 millimeters. The velocities that are encountered in the human circulatory system vary between 1 m/sec in the larger vessels to 10.sup.-4 m/sec in the capillaries. Pressures may vary between 150 mm of Hg in the aorta to nearly zero in the major veins.
The prime mover in the human circulatory system is the heart. In a normal adult at rest, the heart has an output of about 5,000 ml/minute at a mean rate of about 70 beats per minute. During periods of exercise, this output can rise by a factor of five. Assuming a classical lifespan of about 70 years, before the end of its useful life a normal heart will have pumped approximately 200,000 tons of blood through the circulatory system, normally without maintenance.
With recent advances in technology and medical procedures, it is now possible to treat critically ill heart patients in some cases by total replacement of a defective heart with a normal heart from a human donor. Such heart transplants are now well known and have received widespread attention. However, obviously the number of donors who can provide a normal heart for replacement of a defective one in a critically ill patient is extremely limited. This suggests the importance of developing a totally artificial heart for replacement of the natural heart in such patients.
Investigation into the development of an artificial heart that can be satisfactorily transplanted in critically ill patients has been the subject of intensive research in recent years. Efforts in this area are beginning to culminate, with the hope that before long critically ill heart patients will have a chance to enjoy prolonged life by means of an artificial heart transplant.
One of the more promising types of artificial hearts which has been developed consists of an artificial ventricle having a base on which is mounted a semi-rigid urethane polymer chamber which contains prosthetic heart valves to regulate the entry and exit of blood. The blood chamber and base are separated by a flexible diaphragm. The pumping action of the artificial ventricle is controlled by a "heart driver." The function of the heart driver is to impart a pumping action to the flexible diaphragm. This may be done mechanically, hydraulically or pneumatically. For example, in one type of pneumatic unit the heart driver is connected to the base of the ventricle by a flexible plastic tube known as the "driver tube." The driver tube is in turn connected at its origin to the outlet to an electropneumatic valve. The heart driver controls the valve so as to alternately connect the driver tubing to a source of compressed air during systole (i.e., during the phase of the heart's operation where blood is expelled from the heart) and to an exhaust port during diastole (i.e., that phase of the heart's operation where the heart is relaxed and the blood re-enters the heart chambers).
During systole, compressed air is allowed to enter the gas chamber of the ventricle and in so doing exerts a force on the blood contained in the blood chamber of the ventricle via the flexible diaphragm. This force is sufficient to cause the flexible diaphragm to expel the blood through the outflow valve. The duration of the systolic phase of operation is electronically controlled and ends by de-actuation of the electropneumatic valve which then transfers the driver tubing connection from the compressed air source to the exhaust port. This releases the pressure on the flexible diaphragm so that blood can then re-fill the ventricle via the inflow valve.
Recent studies in which either fluid (i.e., pneumatically or hydraulically) driven or mechanically driven types of artificial hearts have been transplanted in animals have yielded promising results, and efforts are now underway to begin limited experimental use of these types of artificial hearts in human patients.
In the past, one of the difficulties arising out of the use of heart replacement with a fluid or mechanically driven artificial heart has been the difficulty of obtaining reliable direct measurements of hemodynamic parameters like pressures, venous return and cardiac output. Knowledge of the cardiac output is useful in monitoring the circulation of a critically ill patient, especially if repeated information can be obtained speedily and easily and with the least possible interference with treatment and recovery of the patient. Especially after total heart replacement, the continuous and reliable measurement of these parameters is the basis for determining whether the artificial heart is properly functioning. It is also the basis for providing automatic control of the heart driver.
There have been several methods and systems employed to measure cardiac output in the prior art; however, all of these have been less accurate than desirable and they also have severe practical problems or risks associated with their use, which in almost every case requires some type of invasive technique.
For example, one widely known method for measuring cardiac output is called the Fick technique, which is performed by measuring the oxygen consumed by the patient by means of a specialized apparatus. Catheters are employed to withdraw venous blood at the right ventricle and arterial blood from the carotid artery. The difference in oxygen content between arterial and venous blood for any given oxygen consumption is an index of cardiac output, which may be calculated from these measured variables.
The Fick method is rather laborious, has an undesirable aspect of requiring insertion of arterial and venous catheters as well as collection of expired gas, and has a measurement accuracy that is only in the range of .+-.10-20% of the actual cardiac output.
Another prior art method for providing flow measurement is called the dye-dilution method. This method involves injecting dye into the blood stream at a point upstream of the ventricle and then sampling the blood downstream of the ventricle. The concentration of dye measured per unit of time in the blood downstream of the ventricle provides an indication of the cardiac output.
Like the Fick technique, the dye-dilution method also suffers from a relatively poor measurement accuracy, in the range of .+-.10-20% of the true value. Moreover, when used with an artificial heart the dye-dilution technique creates an additional problem by virtue of the fact that an artificial heart valve will not fully close when a catheter is threaded through the valve, which is necessary to inject the dye. Therefore, the artificial valve leaks considerably and it is not possible to provide a true and accurate measurement of cardiac flow under these circumstances.
Yet another prior art method is the thermal-dilution technique, which is described in an article entitled "Cardiac Output Measurement By Thermal-Dilution" by Sorenson et al. See Annals of Surgery, Vol. 183, No. 1, January 1976, pp. 67-72. This technique involves placing a catheter via a vein so that its tip passes through the right ventricle to lie in the pulmonary artery. A thermistor is located near the catheter tip. Cool saline solution is delivered into the upper end of the catheter, to emerge in the right ventricle and temperature measurements from the thermistor, at the downstream location, are measured over a given time interval and can be used to provide an estimate of cardiac output.
One problem with the thermal-dilution technique is the fact that changes in temperature of the saline solution occurring between its introduction point and measurement point render accurate detection and other measurement difficult. The technique has an accuracy of only .+-.20% of the actual flow value. Moreover, the thermal-dilution technique suffers from the same problem as the dye-dilution technique when used with an artificial heart, in that an artificial heart valve will not fully close when a catheter is threaded through it to introduce the saline solution into the patient's circulatory system.
Still another prior art method for measuring cardiac output is described in an article entitled "The Measurement of Flow in Intact Blood Vessels" by Roberts. See CRC Critical Reviews in Bioengineering, August 1973, pp. 419-452. Basically, this technique involves the use of an electromagnetic flow meter. The electromagnetic flow meter exploits the fact that a moving electrical conductor generates a voltage when it passes through a magnetic field. In the case of this type of flow meter, the blood acts as the moving conductor. A coil of wire around the blood vessel is used to generate the magnetic field and electrodes applied to the blood vessel are used to detect the voltage generated by the moving blood.
This device works particularly well on a laboratory bench, but experiences serious difficulties when employed in vivo. For example, the electrodes must be firmly applied to the vessel wall; however, any movement of the body can adversely affect this arrangement. The wires connected to the electrodes and coil are fragile, and major surgery is required to put the device into the patient's body. Moreover, once the device is in the patient's body, it cannot be calibrated.
In summary, the prior art systems for providing cardiac flow measurement do not provide the necessary degree of accuracy to be able to provide reliable data which can be used in diagnosing whether an artificial heart is properly functioning or which can be reliably used as the basis for controlling the heart driver. Moreover, as indicated, these prior art techniques have the further serious drawback that they involve invasive techniques which pose additional risk and discomfort to the patient, which is undesirable, particularly in the case of a critically ill cardiac patient.